1. Field of the Invention
Exemplary aspects of the present invention relate to a method and apparatus for illuminating a document. More particularly, exemplary aspects of the present invention relate to a document illuminating unit, using a fluorescent lamp serving as a light source, that efficiently illuminates an original document with a large quantity of light and a low amount of flare, an image scanner utilizing the document illuminating unit, and an image forming apparatus, such as a copier, printer, facsimile machine, and a multi-functional machine including the functions of the copier, printer, facsimile machine, and so forth. The image forming apparatus include the image scanner with the document illuminating unit.
2. Discussion of the Related Art
Related art image scanners included in an image forming apparatus generally employs a xenon lamp, having a diameter of approximately 10 mm, as a fluorescent lamp. The xenon lamp has a brightness smaller than a halogen lamp but has a greater amount of light emission because the xenon lamp has a greater area for emitting visible light rays. The xenon lamp also has higher luminous efficiency with respect to electric power consumption. Further, the amount of light emission of the xenon lamp is substantially proportional to an area covered with fluorescent coating. Therefore, the amount of light emission of the xenon lamp increases when a diameter of a glass tube of the xenon lamp is increased and fluorescent coating is applied to a wider area of the xenon lamp. However, a greater diameter of the glass tube requires a greater size of the document illuminating unit, and a greater size of an image scanner.
FIG. 1 shows a cross sectional view of a main portion of a related art xenon lamp 2 that has generally been used as a light source of a document illuminating unit. In FIG. 1, the xenon lamp 2 includes an opening or aperture 2a, a fluorescent material 2b, a lamp bulb or glass tube 2c, and electrodes 2d and 2e. The glass tube 2c has a transparent and cylindrical shape and a thickness of approximately 0.5 mm to approximately 1 mm. The electrodes 2d and 2e are disposed on a circumference of the glass tube 2c, facing each other. The xenon lamp 2 is filled with xenon gas and the electrodes 2d and 2e are applied with a predetermined amount (several hundred) of an alternating voltage so that light can be discharged from the xenon lamp 2. This discharge causes flows of electrons, which causes the electrons of xenon gas passing through the glass tube 2c to collide with the atoms of xenon gas in the glass tube 2c. This collision can generate ultraviolet rays. When the ultraviolet rays illuminate the fluorescent-coated inner circumference of the glass tube 2b, the fluorescent material is excited to emit visible light rays. The visible light rays are emitted from the aperture 2a to the outside the fluorescent lamp 2.
FIG. 2 shows a related art document illuminating unit utilizing the above-described xenon lamp as a light source to illuminate an original document. The related art document illuminating unit of FIG. 2 includes one xenon lamp and one reflection plate for illuminating its image reading area. A portion of luminous fluxes emitted from the xenon lamp travels through an opening or aperture (not shown) via the reflection plate to illuminate an original document placed on a contact glass. The other portion of luminous fluxes emitted from the xenon lamp directly illuminates the image reading area of the original document on the contact glass. After these luminous fluxes are reflected, the reflected luminous fluxes are directed to an image forming lens via first, second, and third mirrors (not shown) so that they can be ready as image data by an image reading unit, such as a CCD (charge-coupled device) sensor.
Related art reflection plates have a surface with an elliptic shape, a paraboloidal shape, and so forth. When a lamp serving as a light source has a columnar shape, a related art reflection plate corresponding to the lamp has a cylindrical shape.
Further, the reflection plate having a shape of an ellipse has two focal points, and a center of the fluorescent lamp is arranged at one of the two focal points, the focal point located at a far side of the original document.
The above-described related art document illuminating unit suffers from the disadvantage that a high-speed full-color image scanner that has high productivity requires a large amount of light to illuminate a document. To obtain a large quantity of light, however, the xenon lamp 2 needs a greater area coated by fluorescent agent, which requires a greater diameter of the glass tube 2c so as to flow a greater amount of electric current. If the diameter of the glass tube 2c becomes greater in size, the area of the aperture 2a also becomes greater. This causes the reflection plate itself to have a greater size, which contributes to maximization of the document illuminating unit.
Further, since the xenon lamp 2 has a relatively large area of light emission and has the aperture 2a to emit luminous flux, it is difficult to collect the luminous flux to illuminate a document area. For example, as shown in FIG. 2, the luminous flux emitted for illuminating a document area A may illuminate as far as an area B. That is, the luminous flux illuminates a wider area than expected. Therefore, reflecting light rays of unnecessary luminous flux travel via a reflection plate and the xenon lamp 2 to illuminate the original document. Thus, the original document is illuminated again. When the reflecting light rays have has a large quantity of light, an amount of light emission may change according to a density of the original document. The density of an original document is substantially equal to an area coverage rate of the original document, which shows how much image data covers a reading area on the original document. For example, when an original document has a low area coverage, the density of the original document becomes lower. When an original document has a high area coverage, the density of the original document becomes higher. The related art document illuminating unit has a disadvantage that a density value of scanned image data becomes different from a density value of data of an original document, and the reproducibility of image may become worse. Further, the reproducibility of image also becomes worse even when the related art document illuminating unit having a lamp generally with a large quantity of light is used.